A sludge concentrating method commonly applied in a general sewage disposal plant will now be described with reference to the drawings.
FIG. 16 is a flowchart illustrating the sludge concentrating method in the sewage disposal plant.
As shown in FIG. 16, sewage flowing into a primary settling tank 1 is subjected to a solid/liquid separation through a gravity precipitating treatment. Then, supernatant water after the precipitation treatment is fed to a reaction tank 2, where it is subjected to an activated sludge concentration. The sewage thus treated in the activated sludge concentration is fed to a secondary settling tank 3, where it is subjected again to a solid/liquid separation through a gravity precipitating treatment. The resultant supernatant water obtained in the secondary settling tank 3 is fed to a sterilizing tank 4, where it is sterilized. The treated water resulting from sterilization of sewage is discharged to rivers, lakes or sea water.
On the other hand, sedimentation sludge resulting from the primary settling tank 1 and excess sludge resulting from the secondary settling tank 3 are usually mixed. The resultant mixed sludge is fed to a sludge concentrator 5. The mixed sludge fed to the sludge concentrator 5 has usually a solid concentration of about 1% or so. The mixed sludge is subjected to a concentration treatment in the sludge concentrator 5 through gravity precipitation. This results in a concentrated sludge having a solid concentration of about 2 to 3%. The resultant concentrated sludge is stored in a sludge storage tank 6. The sludge stored in the sludge storage tank 6 is dehydrated by a dehydrator 7. Dehydrated cakes are thus obtained.
In an ordinary sewage disposal plant, as described above, gravity precipitation is used for concentration treatment of mixed liquid of sewage flowing down and sludge (hereinafter referred to as “sludge”). More specifically, the sewage flowing down is subjected to solid/liquid separation by gravity precipitation in the primary settling tank 1. The sewage concentrated through activated sludge is subjected to the solid/liquid separation by gravity precipitation in the secondary settling tank 3. The mixed sludge is concentrated by gravity precipitation in the sludge concentrator 5.
The above-mentioned sludge concentrating method based on gravity precipitation requires only small power consumption, thus providing an advantage of a low treatment cost. However, the apparatus for sludge concentration based on gravity precipitation has a large area of installation, and has a problem in that the treatment ability of the apparatus largely depends upon properties of sludge and the water temperature to be treated.
In recent years, particularly, precipitation property of sludge is becoming worse by causes such as increase of organic matters in sludge and putrefaction of sludge transported over a long distance as a result of achievement of intensive disposal of sludge. Consequently, it is the current reality that in the sludge concentrator in a sewage disposal plant conducting intensive disposal of sludge, it is becoming more difficult to ensure a concentrated sludge constantly having a solid concentration of 2% or higher through sludge concentration method based on gravity precipitation.
When a concentrated sludge of a prescribed concentration is unavailable, the amount of mixed liquid of sludge fed to dehydrating equipment in the latter step becomes larger. This not only makes it difficult to operate the dehydrator in stable, but also requires excessive facilities for dehydration apparatus.
For the purpose of solving these problems, mechanical concentrating apparatuses such as a centrifugal separator are being adopted recently, but this method is economically unfavorable because of the large power consumption.
In high-level treatment methods such as removal of nitrogen or phosphorus of which the positive adoption is currently considered with a view to preventing eutrophication of public waters, the reaction tank is operated with a higher solid concentration than in the standard activated sludge process in many cases in order to increase a reaction rate. It becomes therefore necessary to apply a solid/liquid separation treatment to sludge-mixed liquid having high concentration flowing from the reaction tank. In a metropolis in particular, where many secondary settling tanks coping with the standard activated sludge process are already in practical operation, adoption of a high-level treatment requires replacement of the secondary settling tank 3 in FIG. 16, or development of new sludge concentrating means capable of complementing the same.
Demand for a technique permitting concentration of sludge for certain without relying upon gravity precipitation is now increasing as described in the foregoing.
From the above-mentioned point of view, the present inventors developed a sludge concentrating apparatus disclosed in Japanese Unexamined Patent Application Publications Nos. 11-216312, 2000-5506 and 2000-262817.
The present inventors further studied the above-mentioned conventional sludge concentrating apparatus, and obtained the following findings. By adding the coagulant, then slowly transferring the coagulated flock, and uniformly feeding the same in the width direction of filter cloth, it is possible to take large steps toward improving efficiency of concentration of sludge, and they completed the present invention.
That is, the present invention has an object to provide a sludge concentrating apparatus which permits, when disposing of sludge by using the above-mentioned conventional sludge concentrating apparatus, efficient concentrating treatment in a large quantity without depending upon properties of sludge.